"use strict"
class C {
  constructor(x) {
	this.x = x;
  }
  a() {
	console.log("a");
  }
}
class D extends C {
  constructor(x, y) {
	super(x); // 调用父类的constructor(x, y)
	this.y = y;
  }
  b() {
	console.log("b");
  }
}
var obj = new D(1,3);